Specific gravity, also known as the relative density, is calculated by dividing the density of a substance by a reference density. The most common reference density is pure water, making one common definition of specific gravity the ratio of a substance's density to that of pure water.
The density of a substance can be calculated by dividing its mass by its volume. The density of water can change with temperature but is always fairly close to one gram per milliliter. Because of this, a quick estimate of a substance' specific gravity (when using pure water as the reference density) would be the value of its density.
Specific gravity was discovered by Archimedes when he noticed that certain compounds would displace more water than many other compounds when they were both submerged in identical amounts of water. The greater the specific gravity of a compound, the more water it displaces when submerged in pure water. Besides determining the displacement of water, specific gravity can also help determine if an object is going to float or sink in water. If the specific gravity of a substance is less than one, it floats in water, but if its specific gravity is more than one, it sinks.